Semiconductor laser diodes having stable emission spectra are desirable for use in high performance optical communications systems.
External optical cavities have been optically coupled to semiconductor laser diodes to modify the emission spectra of the semiconductor laser diodes (Goldberg et al, IEEE J. Quantum Electron., 1982, QE-18, pp. 555-563; Acket et al, IEEE J. Quantum Electron., 1984, QE-20, pp. 1163-1169; Tkach et al, IEEE J. Lightwave Technology, 1986, LT-4, pp. 1655-1661). The modified spectra are sensitive to the phase of external optical feedback from the external optical cavity. Hence, the phase of the external optical feedback must be controlled in order to stabilize the modified spectra.
Several schemes for controlling the phase of the external optical feedback have been discussed in the literature (Favre et al, Electron. Lett., 1983, Vol. 19, pp. 1046-1048; Ho et al, Third International Conference on Optical Fiber Sensors, San Diego, 13-14 February 1985; Neilsen et al, J. Opt. Commun., 1984, pp. 41; Jain et al, Electron. Lett., Vol. 21. pp. 957-958). Each of these schemes requires one or more photodetectors which add to the cost and complexity of optical transmitters employing such schemes.